Modern technology has progressed to a point where numerous complex electronic accessories are commonly used aboard vehicles such as passenger cars, pleasure boats, or private aircraft. These accessories include, for example, an automatic-answering cellular telephone and telephone answering machine combination. As part of their intended function, these accessories are typically used or left powered-on when the user is away from the vehicle. In such situations, these accessories depend on the vehicle's battery as their power source. Thus, with the use of these accessories comes an increased likelihood of deeply discharging the vehicle's principal battery when the vehicle is left unattended for an extended period of time. This is harmful to the battery. Moreover, since the battery is also used for starting the vehicle, a deeply discharged battery greatly inconveniences a user who is unable to start the vehicle. This problem may also be exacerbated where the battery is not fully charged to begin with, where low ambient temperature debilitates the battery, or where the battery is old and in a weakened condition.
The prior art has taught three types of remedies for this problem. The first of these remedies is simply to interrupt power to the battery-powered accessories when the ignition switch of the vehicle is in the "OFF" position. This method has the clear disadvantage of not allowing any accessories, such as an automatic-answering cellular telephone or telephone answering machine, to be used when the vehicle is unattended. This limitation clearly negates the purposes of these accessories and inconveniences the user of the vehicle.
The second type of remedy taught by the prior art involves a procedure whereby the terminal voltage of the vehicle's battery is measured and compared to a predetermined voltage threshold under which the battery would be in a deeply discharged state. When the terminal voltage of the battery is at or below this threshold voltage, the power to the accessory(s) is interrupted. However, this widely used method has the drawback that, by the time that the corrective action is taken, the battery is already in a state of substantial discharge. Thus, the user of the vehicle may still be inconvenienced where the battery is not left with sufficiently charged to start the vehicle.
The third type of remedy taught by the prior art involves the use of a timer. By this method, the power to the accessory is interrupted after a predetermined period of time following the switching of the vehicle's ignition to the "OFF" position. However, this remedy accommodates only a single level of current drawn by the battery-powered accessory(s). In other words, this method does not account for current draws which may differ between accessories or between the modes of operation of a single accessory, and which may therefore discharge the battery at different rates. For instance, a cellular telephone and answering machine may draw little current when there is no incoming call. In this state, these accessories may remain powered-on for a relatively longer period of time without deeply discharging the battery. On the other hand, a large number of incoming calls, representing a state of heavy use, would result in a substantially higher average current draw, and therefore a substantially higher battery discharge. This state would require that the power to the accessories be interrupted after a relatively shorter period of time to conserve a sufficient amount of charge in the battery. Thus, the dependence of this method on the presupposition of constant current renders this method very inefficient in light of the use-dependency of the current draw of various accessories.
There is an advantage, of course, to a method of protecting the battery of a vehicle from deep discharge due to the operation of battery-powered accessories whereby the method conserves a sufficient charge in the battery to start the vehicle while permitting such accessories to operate for the maximum time commensurate with the actual rate of energy consumption or current draw of the accessories.
A further advantage is gained where the battery-discharge-protection method adapts to external factors. These external factors may include outside ambient temperature which affects the efficiency of the battery, the age and condition of the battery, or the initial state of charge of the battery. This feature prevents the user from inadvertently bypassing the battery discharge protection method by disconnecting and reconnecting the accessory, by turning the ignition switch to the accessory position without starting the vehicle, or by not allowing the vehicle to run for a sufficient amount of time to fully recharge the battery.